1. Field of the Invention
The present invention relates to a wide-angle zoom lens system, for single lens reflex cameras, which has an angle-of-view (=2.omega.) of more than 90.degree. at the short focal length extremity.
2. Description of the Related Art
In a single lens reflex camera, it is necessary to maintain a long back focal distance because a quick-return mirror is positioned behind the photographing lens system. For this reason, a retrofocus lens system having a negative lens element and a positive lens element, in this order from the object, is generally employed for a wide-angle lens system. However, in a retrofocus lens system, it is extremely difficult to correct field curvature in the vicinity of the short focal length extremity. Generally, in a zoom lens system where a negative lens group precedes a positive lens group, each lens group has to be provided with stronger power in order to make an overall length of the lens system shorter while securing a long back focal distance and maintaining the diameter of lens elements smaller.
However, if the power of each lens group is increased, optical deterioration on the wide-angle side occurs at the periphery of the image forming area. In other words, the occurrence of field curvature, astigmatic difference and distortion becomes noticeable. More concretely, in order to prevent field curvature on the wide-angle side which tends to become larger, at a periphery of an image, in the positive direction, field curvature at a lower image height has to be largely bent in the negative direction. Field curvature at a periphery of an image on the telephoto side corresponds to the tendency of field curvature at the lower image height on the wide-angle side, that is, the field curvature at a periphery of a image on the telephoto side is largely bent in the negative direction. As a result, in such a zoom lens system, field curvature extremely changes from the wide-angle towards the telephoto-angle.
Due to the above-mentioned problems, in a zoom lens system, particularly, in a wide-angle zoom lens system in which the angle-of-view at the short focal length extremity is more than 90.degree., for the purpose of miniaturization and in order to correct aberrations other than field curvature, field curvature has to be allowed to remain to some extent.